1. Field of the Invention
This invention relates to color cathode ray picture tubes having a foil tension mask, and is addressed specifically to an improved apparatus and process for manufacturing the shadow mask component.
The use of the tension foil mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high-brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate. Although it is under high tension, the tensioned foil mask will dome, but negligibly so in comparison with the curved mask. Its relative immunity to doming provides for greater brightness potential while maintaining color purity.
The tensioned foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen consisting of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about one mil thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three electron beams generated by the electron gun lands only on its assigned phosphor deposits.
2. Prior Art
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method for processing a color cathode ray tube having a thin foil mask sealed directly to the bulb. Included in this disclosure is a description of the sealing of a foil mask between the junction of the skirt of the faceplate and the funnel. The foil mask is noted as having a greater thermal coefficient of expansion than the glass to which it is mounted, hence following a heating and cooling cycle in which the mask is cemented at the funnel-faceplate junction, the greater shrinkage of the mask upon cooling places it under tension. The mask is shown as having two or more alignment holes near the corners which mate with alignment nipples in the faceplate. The nipples pass through the alignment holes to fit into recesses in the funnel.
There have been a number of disclosures of tensed foil masks and means for applying tension to the mask and maintaining it in tension. Typical of these is the disclosure of Law in U.S. Pat. No. 2,625,734, which addresses the construction of a taut, planar, foraminous mask, and the mounting of the mask and target (the screen on the faceplate) as a unitary assembly within the envelope. The thin metal is clamped in a frame, and the mask is heated and placed under screw tension. Upon cooling, the metal contracts and the mask is thus rendered taut and held in tension by the frame.
In U.S. Pat. No. 4,069,567 to Schwartz, assigned to the assignee of the present invention, there is disclosed a method useful in the manufacture of a color cathode ray tube of the type with a phosphor screen, and having spaced therefrom a tensed color selection electrode. In a preferred execution, the method comprises selecting for the electrode a material which has a significantly greater coefficient of thermal expansion than that of the holder. The electrode and the holder are externally heated together, as by an oven, while the electrode is tensed. Simultaneously therewith, a selective auxiliary heating of the electrode is induced, as by passing an electrical current through the electrode, or by RF heating, such that the electrode is heated to a predetermined elevated temperature signifi-cantly greater than that of the holder so that the electrode is caused to thermally expand a greater amount than the holder. The electrode is then affixed to the holder. Finally, the electrode and holder are cooled to room temperature so as to hypertense the electrode due to greater coefficient of thermal expansion of the material from which it is made.
A thin, perforated diaphragm used as a shadow mask was used in an early color picture tube called a "Colortron." The mask, which is round, is used as an optical stencil for use in photographically depositing color phosphors on an associated screen. The mask, when mounted on a frame, is said to be lightweight and self-supporting, and the use of the frame facilitates insertion and removal during the photoscreening process. ("The CBS Colortron: A Color Picture Tube of Advanced Design". Fyler et al. IRE R583.6. January 1954.)